The Australian Transport Safety Bureau (ATSB) is investigating how two passenger jets came too close to each other while circling near the Gold Coast airport last month.

The ATSB says more than 300 passengers were aboard the Virgin Australia and Qantas 737s that were travelling from Melbourne to Brisbane on July 29.

Planes must be separated by about 300 metres vertically and nine kilometres laterally, but it is unclear which of these rules was breached.

Australian and International Pilots Association spokesman Captain Richard Woodward says it is "reasonably rare" for passenger planes to get too close to each other in the air.

He says it is possible there was a communication breakdown.

"There is the off-event - a classic one - where if you're going from one country to another and there's a handover between the two countries and there's confusion about the altitude that's been assigned to the crew," he said.

Virgin says there was never any safety risk and crews were following the instructions of air traffic controllers at the time.

A spokeswoman for Qantas declined to be interviewed.

The ABC has requested an interview with the ATSB, which is not expected to release its report before next June.